At large spatial scale \(\lambda =500m^{-1}\) , when \(b=0\) , we can see the bell-shaped momentum spectrum in the negative region with oscillation which can be understood as the interference effect between two separated pair production events that happen at different times [1]}, see Fig. REF (a). For small chirp, momentum spectrum shrinks and shifts to positive direction, see solid line in Fig. REF (b). For large chirp, the bell-shaped momentum spectrum is replaced by strong oscillation, see Fig. REF (c) and (d). In the next section, we will discuss some of these oscillation effects in the semiclassical picture.
